It has become increasingly common to treat a variety of medical conditions by introducing a drug releasing medical device into the vascular system. For example, medical devices used for the treatment of vascular diseases include drug eluting stents. There is an increasing demand for better coating methods to control the dose of therapeutic agent and to improve drug distribution and uniformity in the coatings of these medical devices.
Methods for coating a drug eluting stent (DES) have been developed in recent years. The stent is coated with a polymer into which drug is impregnated. Methods for coating drug eluting stents include dipping in, or spraying with, the coating solution or composition. The coating composition often includes a solvent, a polymer dissolved in the solvent, and a therapeutic agent dissolved or dispersed in the coating composition. The composition is then applied to the stent by spraying the composition onto the stent or by dipping the stent in the coating composition. The solvent is allowed to evaporate, leaving a coating of the polymer and therapeutic drug on the stent surfaces. These methods are useful for coating discontinuous surfaces, such as that of a stent. The surfaces outside, inside, and in between struts of the stent can be coated by these methods. In both spraying and dipping coatings, the amount of coating transferred is not precisely controlled and has to be independently quantified for dose verification. Most of the coating does not spray onto the medical device. Thus, the amount of the coating on the medical device is less than the amount of coating that is sprayed. The amount of coating on the medical device in the dipping coating depends on solvent, solution, concentration, and adhesive property of coating to medical device. The dose or load of drug coated on the stent may be controlled by weighing the stent after the coating is dried, since a stent is a small metal implant that can easily be placed on a scale. A precise balance can be used to measure the total dose of drug coated on the device. An important limitation of these methods is that the drug dose cannot be controlled if the medical devices cannot be weighed precisely, or if the weight of the coating layer is negligible relative to the weight of the device, such as a balloon catheter. A balloon catheter typically may be an assembly of long plastic tubes that weighs, for example, approximately 10 to 20 grams. The weight of the drug coating (typically 0.1 to 10 mg) is therefore well within the measurement error of the weight of the balloon catheter itself.
Non-stent based local delivery systems, such as balloon catheters, have also been effective in the treatment and prevention of restenosis. The balloon is coated with an active agent, and when the blood vessel is dilated, the balloon is pressed against the vessel wall to deliver the active agent.
The current method for drug coating of a balloon catheter is dipping or spraying. The coating layer formed by dipping or spraying is not uniform on the surface of the balloon, and the drug is not uniformly distributed in the coating layer overlying the balloon surface. Furthermore, the dose of the drug deployed on the device after dipping or spraying is not consistent and in some cases may vary from as much as 0.5 to 11 μg/mm2, or as much as 300%, from balloon to balloon or from one region of the balloon surface to another. In the case of spray coating, large amounts of sprayed drug will not land on the surface of the balloon catheter and the amount of the drug on the balloon catheter is less than the amount of drug sprayed. In the case of dip coating, it is also very difficult to load a large amount of drug on the balloon, even with multiple dips, because drug already on the balloon may dissolve away during subsequent dips. In both situations, sections of the device that are not desirable to coat must be masked.
Thus, there is still a need to develop an improved method and apparatus for coating highly specialized medical devices. There is still a need to develop improved methods for precisely measuring and controlling the concentration or dose of drug on the surface of coated medical devices. There is a need that the amount of drug dispensed is the same as that on the surface of the medical devices, especially balloon catheters. Furthermore, there is still a need to improve the uniformity of drug distribution in the coating layer and the uniformity of the coating on the surface of the medical device.